202 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



The names of this genus and Crassatellina were suggested rather by 

 their general resemblance to some of the forms that, have been referred to 

 those genera, than from any certainty that they are very nearly related to the 

 same. 



A r c o p a g c 1 1 si mactroides, Meek. 



Plate 2, figs. 4, a, b, c, d. 



Arcopagella mactroides, Meek (1871), iu Hayden's Sec. Aun. Report United States Geological Survey of 

 the Territories, 309, with wood-cuts A aud B. 



Shell longitudinally subovate ; width or height about two-thirds the 

 length, rather compressed or moderately convex ; pallia! margin forming a 

 regular semi-elliptic curve from end to end ; anterior margin narrowly 

 rounded, with the most prominent part near the middle ; posterior border 

 more narrowly rounded than the anterior, particularly below, where there 

 seems to be the faintest possible tendency to form a flexure or fold ; beaks 

 moderately prominent, located very nearly centrally ; dorsal outline sloping 

 almost ecjually before and behind the beaks, but with the anterior slope 

 slightly concave in outline above, and the posterior a little convex ; muscular 

 impressions faintly marked and rather narrow-subovate ; pallial line with its 

 rather shallow, broadly-rounded sinus directed very obliquely forward and 

 upward. Surface apparently with only fine lines of growth. 



Length of one of the larger specimens, 0.78 inch ; height, 0.53 inch ; 

 convexity, about 0.26 inch. 



This shell will be readily distinguished from the Tellinas of our Creta- 

 ceous rocks, by its shorter, slightly more convex, subtrigonal, or subovate, 

 nearly equilateral form, even where its hinge cannot be seen. In some 

 respects, it looks externally like a compressed and depressed Mactra. I am 

 unacquainted with any Tertiary species with a similar hinge. 



Locality and position. — Twelve miles southwest of Salina, Saline County, 

 Kansas ; in the Dakota group of Upper Missouri Cretaceous. Specimens 

 received from Prof. B. F. Mudge, of the Kansas Agricultural College. 



Arcopagella! macrodonta, Meek. 



Plate 1, fig. 2. 



Shell subtrigonal, compressed, with height equaling about three-fourths 

 the length; dorsal margin sloping rather abruptly, and nearly equally from 



